The present invention relates to ball nose tools, preferably ball nose end mills, with improved properties.
Ball nose end mills are advanced tools which have to manage the most changing working conditions at the same time. The most distinguishing feature of ball nose end mills are good cutting properties which are required in the form of wear resistance and the ability to resist high temperatures at the periphery, i.e., at full nominal measure, and simultaneously as the tool is required to generate chips and function as a cutting tool in the center where the cutting speed approaches zero. Between the periphery and the center, there is a continuous change of the cutting speed through all possible built up edge areas, etc.
Ball nose end mills are often used in difficult operations where demands are very high in terms of surface finish, for example, the aerospace industry when milling wing spars, etc. In this application, no unevenness and notches whatsoever may be tolerated which later may be able to cause failure. Another large area of application is the finishing of molding tools where demands on high surface finish and accuracy to shape with simultaneous high productivity and predictable long tool life are especially great. Further, the tool may not be exchanged during the machining operation which could result in worse precision of the manufactured part as the tool wears.
The geometries which normally exist on today's conventional tools, whether they are manufactured of tough materials such as high speed steel or brittle, more wear-resistant cutting materials such as cemented carbide, often have a negative rake angle and small chip room in and near the center of the ball nose end mill. The drawbacks with this tool are primarily high cutting forces and insufficient space for the chip in the center which results in uneven wear and risk of edge damages.
Through U.S. Pat. No. 4,145,213, a material is known containing 30-70 volume % submicron hard constituents in a metallic binder phase. This material has superior wear resistance compared to advanced high speed steel and can therefore be considered as having properties between cemented carbide and high speed steel. Further, U.S. Pat. No. 4,618,540 discloses superior compound tools made with, e.g., the above material placed in the areas subjected to high cutting speed, and with high speed steel in the center for drilling applications where there is the zero speed problem. The purpose with this later invention was partly with the aid of a tougher core, to obtain a tool with better macro toughness and partly to achieve better grinding economy since the hard constituent rich material is considerably, more difficult to grind than, for example, high speed steel. In addition, the zero speed problem and built up edge formation areas were experienced as problematic for the submicron hard material with 30-70 volume % hard constituents.